1. Field of the Invention
This invention relates generally to apparatus for recording and/or reproducing a digital signal and more particularly to an apparatus for recording and/or reproducing a digital signal in a so-called digital audio tape recorder and the like.
2. Description of the Prior Art
So-called digital audio tape recorders (DAT) that are now under development are of the fixed or stationary head type (S-DAT) or of the rotary head type (R-DAT). In the stationary head type digital audio tape recorder, a digital audio signal is recorded in many tracks parallel to the longitudinal direction of a tape by using a multi-track stationary head. In the rotary head type digital audio tape recorder, a digital audio signal is recorded in tracks skewed relative to the longitudinal direction of the tape by rotary heads of the helical scanning system.
FIG. 1 shows a tape format used in the above mentioned rotary head type digital audio tape recorder. Examples of various dimensions in FIG. 1 will be indicated below.
______________________________________ A tape width 3.81 mm W effective recording width 2.61 mm Vt tape speed 7.20 mm/s L track length 23.5 mm P track pitch 12.0 .mu.m .theta. track angle 6.degree. 22' .alpha. azimuth angle of head gaps .+-.20.degree. ______________________________________
In a recorder for use with the above defined tape format, a rotary head drum about which the tape is wrapped has a diameter of 30 mm and is rotated at a speed of 2000 rpm, by way of example. Further, the recording speed (Vh in FIG. 1) is selected to be 3.13 mm/s and the tape wrapping angle is selected to be 90.degree..
If, for example, music is recorded by the rotary head type digital audio tape recorder, the recording is carried out in accordance with a track format and a block format shown in FIG. 2 and FIGS. 3A and 3B, respectively. At such time, positional information, such as, a program number, a track number, a time code (code indicating the time, in minutes and seconds, from the starting point of the program) and the like, and sub-codes containing other necessary auxiliary data are also recorded.
As FIG. 2 shows that one track, which is recorded by one rotary head, is formed of 196 blocks, in which a PCM (pulse code modulated) audio data recording area comprised of 128 blocks is substantially centered in respect to the length of the track. An ATF (automatic track following) area comprised of 5 blocks and in which a pilot signal for the tracking operation is recorded is provided at opposite sides of the PCM audio data recording area, and sub-code areas 1 and 2 each comprised of 8 blocks for recording sub-code data are provided outside the ATF areas.
The PCM area and the sub-code areas 1 and 2 are provided with respective PLL (phase locked loop) areas of 2 blocks each for synchronization. GAP areas of 3 blocks each are provided between the PCM area and the ATF areas and between the latter and the sub-code areas 1 and 2. If the timing for switching the recording mode and the playback mode is matched with the gap areas between the PCM area and the sub-code areas 1 and 2, it becomes possible to independently record the PCM audio signal or the sub-code signal. The track format of FIG. 2 is completed by margin areas of 11 blocks at the opposite ends of the track and by post amble areas of 1 block situated between sub-code area 1 and the adjacent GAP and between sub-code area 2 and the adjacent margin area. The numerals on FIG. 2 will be seen to indicate the numbers of blocks that constitute the respective areas.
As shown in FIG. 3A, each block is formed of 288 bits and includes an 8-bit block synchronizing signal, an 8-bit identifying code (hereinafter simply referred to as ID code), an 8-bit block address code, an 8-bit parity code and 256-bit data (PCM data or sub-code data) which are sequentially located in the order given. The MSB (most significant bit) of the block address code is assigned "0" in each block containing PCM data and "1" in each block containing sub-code data as shown in FIG. 3B.
The ID code in each block containing PCM data is used to indicate the use of audio data, emphasis characteristic, quantization characteristic, sampling frequency, the number of channels, the presence or absence of editing of the PCM audio signal and the like. In each block containing sub-code data, the presence or absence of editing of the sub-code data, a recording start point (head) of the recorded program, interval to be skipped in the playback mode and so on are assigned to the MSB,2SB (second significant bit), 3SB (third significant bit), ..., of the 8-bit ID code, respectively.
At the start portion of each program, the 2SB of the ID code in a block containing sub-code data becomes "1" for 10 seconds and is used as a program start signal. The 2SB of the ID code in a block containing sub-code data is recorded as "0" in portions other than the start portion of each program.
Further, the interval on the tape in which the 3SB of the ID code in a block containing sub-code data becomes "0" is normally reproduced in the playback mode, whereas, in the interval in which the 3SB of the ID code of each such block becomes "1", the program recorded in that interval is considered to be unnecessary and is skipped in the playback mode. Such 3SB of the ID code is referred to as a "skip bit" hereinafter.
When a program, for example, music recorded on the magnetic tape, is reproduced and listened to, there may be a non-recorded (blank) portion on the tape or a portion in which unnecessary information is recorded. In that case, it is desirable that the blank portion or unnecessary information be skipped and that the music be reproduced and listened to without intermission.
In order to skip the blank portion, it is sufficient that the blank state or interval be detected by sensing the level of the reproduced audio signal, and when this blank state or interval lasts, for example, more than 10 seconds, the magnetic tape is transported at a high speed, for example, 16 times the normal tape speed.
Alternatively, in order to skip the tape portion containing unnecessary information, it has been the practice, by way of example, to continuously record the above-mentioned skip bit of the sub-code along the entire interval on the magnetic tape to be skipped, as indicated at (SKIP ID) on FIG. 4. When this skip mark (SKIP ID) is detected in the playback mode, the magnetic tape is transported at the high tape speed, for example, 16 times as high as the normal tape speed.
However, in the case of the conventional system in which the skip bit is continuously recorded along the interval of the magnetic tape which is to be skipped, only the interval in which the skip bit exists is skipped so that the skip interval is cleared. When the skip bit is recorded on the sub-code area in the so-called after-recording mode, an after-recording time corresponding to the time interval to be skipped is required. In other words, if there is a portion of the recorded program to be skipped which continues for a substantial period, for example, 3 minutes, it is necessary that the start thereof be specified at a certain point, the end thereof be specified at a different point, and the interval between the start and end points be recorded in the after-recording mode for 3 minutes to record the skip bit therealong. Further, when the recorded skip bit is to be erased, the same time period is required.